The present invention relates to the transfer of electromagnetic energy between coils, and more particularly, to an external device secured to the skin of a patient, having a coil for transmitting power and/or data to an implanted device and/or to a replenishable power source associated with the implanted device.
The present invention will be described in the context of a cochlear implant system; however, it should be understood that it is also useful for other medical devices such as neuromuscular stimulators, implantable pumps, and other implantable devices that are designed to be surgically inserted within a patient's body to carry out a medically related function for an extended period of time. Indeed, the present invention can be used in any device having an implantable portion including an implanted coil and an external portion containing an external coil for placement on the skin of the patient in alignment with the implanted coil. In a cochlear implant system, the portion of the system containing the external coil is placed on the head and is referred to as the “headpiece”. The term “headpiece” will be used herein to refer to the portion of the system containing the external coil, regardless of where it is used on the body.
A common way of achieving alignment between an external coil and an implanted coil is to employ a magnet in both the headpiece and the implanted device. The magnetic attractive force associated with such magnets holds the external coil in close proximity to the implanted coil and provides the desired alignment between the coils for efficient electromagnetic coupling, which may be RF, inductance, optical, or the like.
However, a known problem is that the pressure of the headpiece against the skin may cause skin irritation and/or tissue damage due to blood flow restriction. The issue is further compounded by variable skin curvatures resulting, in part, from the shape of the implanted device and variable skin thicknesses among patients. Because of this, force may be applied over a small area of the skin flap, resulting in very high local pressures, which can lead to irritation, soreness, redness, and even necrosis of the skin flap tissue or extrusion of the implanted device. Reducing magnetic forces to address the pressure issues has the undesirable effect of poor headpiece retention.
Attempts have been made to improve headpiece comfort and retention by providing a textured elastomeric material at the periphery on the skin side surface of the headpiece, or providing a ring or disk of textured rubber having adhesive on one side to stick to the skin side surface of the headpiece. U.S. patent application Ser. No. 10/741,411, filed Dec. 19, 2003, which is incorporated herein by reference, discloses a shell for covering a headpiece in which portions of the shell that come into contact with skin are covered with or made of a material that is comfortable against the skin; a material such as fabric or foam or other material with similar qualities may be used to maintain a comfortable feel against the skin in moist environments, such as when a user sweats. Other headpiece designs have used an open antenna coil design having a ring of rigid plastic bumps at the perimeter on the skin side surface.
In view of the above, what is needed is a headpiece that can transmit power and/or data transcutaneously to an implanted device, wherein such headpiece is readily attachable to the skin in close proximity to the implanted device, easily and reliably retained in proper alignment with the implanted device, and does not promote skin erosion, irritation, or discomfort, even with minor changes of the skin that commonly occur.